What should the imam of a mosque do if he hears that the mosque was built on usurped land, and the mosque's owner claims the land was abandoned, with doubt regarding the accuracy of this claim?

The answer to this question depends on whether the aforementioned land was usurped or not. The safer course of action is to avoid praying in this mosque to دفعًا للشبهات (avert doubts), especially since its owner admitted that the land belonged to other people. The departure of the land's owners does not permit its development or ownership, no matter how long the time has passed, as scholars have unanimously agreed that what is known to be owned by someone without interruption cannot be revived by anyone other than its rightful owners.